1. Field of the Invention
The present invention relates to a high-frequency circuit which is applicable to a high-frequency module utilizing a radiofrequency in the microwave or millimeter range. More particularly, the present invention relates to a high-frequency circuit which effectively reduces radiation loss associated with a high-frequency signal.
2. Description of the Background Art
A rapid increase in the number of users of wireless devices in recent years underlines the need for an ability to utilize the millimeter range as a new frequency resource. There have been studies conducted to realize millimeter-range distance measuring devices in anti-collision radar for automobiles or the like, in an attempt to take advantage of the short wavelength of the millimeter range. However, before practical applications of millimeter range devices can be realized, the ability to mass-produce low cost and compact high-frequency circuitry will generally be required.
In order to enable mass-production of low cost and compact high-frequency circuitry, various high-frequency packages have been proposed. For example, a high-frequency package has been proposed in which a connection terminal is formed on a lower face of a package where a signal strip line has been taken out by utilizing a throughhole conductor or the like which penetrates a dielectric substrate, such that the package can be surface-mounted, by solder reflow technique, onto the wiring provided on an external circuit substrate.
FIG. 9A is a schematic cross-sectional view illustrating a conventional high-frequency package having been surface-mounted to an external circuit substrate. FIG. 9B is a view illustrating a wiring pattern of conductors formed on an upper face of a dielectric substrate 110. FIG. 9C is a view illustrating a wiring pattern of conductors formed on a lower face of the dielectric substrate 101.
In FIG. 9A, the high-frequency package comprises a high-frequency element 110, a dielectric substrate 101, and a cover 109. The high-frequency package is surface-mounted on an external circuit substrate 113. As shown in FIG. 9B, on the upper face of the dielectric substrate 101, a ground conductor layer 104, two signal strips 102a, and a ground conductor region 104b are formed. As shown in FIG. 9C, on the lower face of the dielectric substrate 101, two signal strips 102b, two ground strips 103 which are disposed so as to leave a predetermined space from the signal strips 102b, and a ground conductor region 104c are formed. The signal strips 102a, the ground conductor layer 104, and the ground conductor region 104c together constitute a grounded coplanar waveguide structure. The signal strips 102b, the ground strips 103, and the ground conductor layer 104 together constitute another grounded coplanar waveguide structure. As used herein, a “strip” refers to a wiring piece of conductor.
One end of each signal strip 102a is connected to the high-frequency element 110 via a wire 111. The wire 111 may be a ribbon or the like. The high-frequency element 111 may be mounted facedown, via conductor bumps. In other words, the high-frequency element 110 may be mounted through wireless bonding, e.g., flip chip mounting. The other end of each signal strip 102a is connected to one end of a corresponding signal strip 102b, by means of a through-via for connection purposes (hereinafter simply referred to as a “through-via”) 112 which penetrates the dielectric substrate 101. Thus, a high-frequency signal which is output from the high-frequency element 110 or a high-frequency signal which is input to the high-frequency element 110 is transmitted via the wires 111, the signal strips 102a, the through-vias 112, and the signal strips 102b, without being grounded. Note that “through-via” is synonymous to “through conductor” for the purpose of the present specification.
On the upper face of the dielectric substrate 101, the ground conductor region 104b is disposed directly under the high-frequency element 110, so as to be electrically connected to the ground conductor layer 104. Via a plurality of through-vias 104d penetrating the dielectric substrate 101, the ground conductor region 104b is connected to the ground conductor region 104c formed on the lower face of the dielectric substrate 101. The ground conductor region 104c is electrically connected to the ground strip 103. Thus, a high-frequency ground is provided in the ground conductor region 104d. An arbitrary number of through-vias 116z are formed between the ground conductor layer 104 and the respective ground strips 103. The through-vias 116z electrically connect the ground strips 103 to the ground conductor layer 104, whereby a better high-frequency grounding ability is provided.
FIG. 10A is a view illustrating an exemplary wiring pattern of conductors formed on an upper face of the external circuit substrate 113. FIG. 10B is a view illustrating an exemplary wiring pattern of conductors formed on a lower face of the external circuit substrate 113.
The external circuit substrate 113 is a substrate on which the high-frequency package is to be surface-mounted. As shown in FIG. 10A, on the upper face of the external circuit substrate 113, two signal strips 114, two ground strips 116, and a ground conductor region 116b are formed. Interspaces are provided between each signal strip 114 and the ground strips 116. As shown in FIG. 10B, on the lower face of the external circuit substrate 113, a ground conductor layer 115 is formed.
Each signal strip 114 is electrically connected to a corresponding signal strip 102b via solder 117. Each ground strip 116 is electrically connected to a corresponding ground strip 103 via solder 117.
The ground conductor region 116b is disposed so as to come directly below the high-frequency element 110. The ground conductor region 116b is electrically connected to the ground conductor region 104c via solder 117. The ground conductor region 116b is connected to the ground conductor layer 115 by means of through-vias 116d penetrating the external circuit substrate 113. As a result, a high-frequency ground is provided in the ground conductor region 116b. An arbitrary number of through-vias 116 are formed between the ground conductor layer 115 and the respective ground strips 116. The through-vias 116 electrically connect the ground strips 116 to the ground conductor layer 115, whereby a better high-frequency grounding ability is provided.
Due to the aforementioned strip line structure, the external circuit substrate 113 functions as a grounded coplanar waveguide in which a high-frequency signal which is output from the high-frequency element 110 or a high-frequency signal which is input to the high-frequency element 110 can be transmitted without being grounded. Note that the ground conductor layer 115 may be provided inside the external circuit substrate 113. Further note that the external circuit substrate 113 will function as a microstrip line if the ground strips 116 are not provided.
Based on the above-described structure, which allows the high-frequency element 110 to be mechanically and electrically connected to the dielectric substrate 101, a compact high-frequency package is provided. Since signal strips are taken out on the lower face of the high-frequency package, it is easy to surface-mount the high-frequency package on the external circuit substrate. Thus, by employing the above-described high-frequency package, it is possible to provide low cost and compact high-frequency circuitry, with good mass producibility.
However, when the high-frequency package having the above structure is employed for transmitting a high-frequency signal, e.g., a signal in the millimeter range, losses may occur in various places. Therefore, the high-frequency package must be designed so as to minimize transmission loss of the high-frequency signal.
FIG. 11 is a cross-sectional view of the dielectric substrate 101 shown in FIGS. 9B and 9C, taken at line A-B. As shown in FIG. 11, the through-vias 116z are arranged in opposing rows astride the signal strips 102b. The ground strips 103, the ground conductor layer 104, the through-vias 116z, and the signal strips 102b together constitute a transmission line such as a grounded coplanar waveguide.
In a transmission line such as the grounded coplanar waveguide shown in FIG. 11, a waveguide surrounding the signal strips 102b is created by the ground strips 103, the ground conductor layer 104, and the through-vias 116z connecting therebetween. When such a waveguide is created, the high-frequency package must be designed so that transmission does not occur in a waveguide mode (i.e., a mode of transmission via the waveguide) at a frequency of the high-frequency signal to be transmitted. Otherwise, at each frequency of the high-frequency signal to be transmitted, the fundamental transmission mode (hereinafter simply referred to as the “transmission mode”) will be converted to the waveguide mode, thereby resulting in transmission losses.
It is known that the waveguide mode can be suppressed by prescribing a distance W between a pair of opposing through-vias 116z to be half of an effective wavelength that corresponds to a designed frequency within the dielectric substrate 101. Assuming that the dielectric substrate 101 has a dielectric constant ε, the effective wavelength of electromagnetic waves in the dielectric substrate 101 can be calculated by dividing a wavelength of the electromagnetic waves in a free space by ε1/2.
M, ITO et al., “Analysis of Millimeter-wave Packaging Structure Using Electromagnetic Simulator and Its Application to W-band Package”, technical report of The Institute of Electronics, Information and Communication Engineers, ED2000-154 MW2000-107 (2000-09), pp.55–60 (hereinafter referred to as “Publication 1”) shows an example design which takes waveguide mode suppression into consideration. In the example design described in Publication 1, a dielectric substrate having a dielectric constant of 7.5 is used, such that the distance between a pair of through-vias across a signal strip is 0.5 mm at the minimum. As illustrated in FIG. 6 of Publication 1, in the case where the design example of Publication 1 is used, deteriorations in the transmission characteristics occur in the neighborhood of 100 GHz to 120 GHz. The minimum opposing distance of 0.5 mm between a pair of through-vias is equivalent to half of the effective wavelength at a frequency of about 102 GHz. Publication 1 attributes such characteristics deteriorations to an increased loss due to a parasitic waveguide mode. In contrast, it can be seen that the transmission characteristics do not deteriorate in a frequency range up to about 90 GHz by using the opposing distance exemplified in Publication 1. Thus, from the description of Publication 1, it can be seen that the waveguide mode can be suppressed if the opposing distance W between each pair of through-vias is prescribed to be half of the effective wavelength corresponding to the designed frequency.
Moreover, radiation losses occurring at connections between signal strips and an external circuit substrate are also problems. At such connections, a high-frequency signal which has been transmitted in the fundamental mode inclines toward a parallel plane mode (which is a higher-order mode) due to an overlap between the ground conductor layer of the connection terminal and the ground conductor layer of the external circuit substrate, thus causing radiation loss.
Japanese Patent No. 3046287 (hereinafter referred to as “Publication 2”) describes an exemplary method for reducing the aforementioned radiation loss. Specifically, Publication 2 proposes removing a portion of the conductor opposing the signal strips from part of the ground conductor layer of the connection terminal, to reduce radiation loss. Based on such a structure, the overlap between the ground conductor layer of the connection terminal and the ground conductor layer of the external circuit substrate is reduced, whereby the parallel plane mode is suppressed. As a result, a high-frequency package which can reduce radiation losses at connections can be realized.
Publication 1 also discloses an exemplary method of reducing radiation losses at connections. Publication 1 includes a detailed discussion of a parallel plane mode which is induced by an overlap between the ground conductor layer of the connection terminal and the ground conductor layer of the external circuit substrate. In Publication 1, at a connection boundary of the ground conductor layer of the connection terminal that lies closest to the substrate, a semicolumnar shaped connection conductor that penetrates through the end face is formed so that proper short-circuiting will occur all the way up to the ultrahigh-frequency band. By forming such a connection conductor that penetrates through the end face, the parallel plane mode is suppressed, whereby radiation loss can be reduced.
However, the above-described conventional techniques cannot attain complete elimination of transmission losses, and would present further problems in practice.
For example, the high-frequency package disclosed in Publication 2 has the problem of an increased module size. Downsizing is an essential requirement in any present-day high-frequency device; however, the high-frequency package disclosed in Publication 2 fails to satisfy this need. In the high-frequency package disclosed in Publication 2, a portion of the ground conductor layer formed closest to an end face of the dielectric substrate is removed. Such a partial removal of the ground conductor layer means that, in view of possible influences on the high-frequency transmission characteristics, it is undesirable to provide a cover, composed of a metal, ceramic, resin, or like materials, above the removed portion. For example, consider a case where a resin substrate is used as a dielectric substrate, under wiring process rules such that wiring can only be provided in portions at least 100 micrometers away from ends of the substrate and that the through-vias have a land diameter of 600 micrometers. Removing portions of the ground conductor layer between through-vias in this case would mean removing an area which is at least 700 micrometers long or more for each through-via existing at an end of the substrate. In this case, since one is prohibited from providing a cover over each of such areas which are 700 micrometers long or more, the size of the high-frequency package will have to be increased, given that the size of the MMIC to be mounted is no more than about one square millimeter. Therefore, the method described in Publication 2 cannot be adopted in whole.
The high-frequency package disclosed in Publication 1 has problems in terms of reliability and reproducibility because, when producing a high-frequency circuit by using a resin substrate or a high temperature-sintered ceramic substrate, etc., it would detract from reliability and reproducibility to form the through-vias in such a manner that the interior is exposed at the ends of the substrate.